Why Do Rockets Need So Much Fuel If, In Space, There Is Nothing to Slow Them Down?

The propellant is primarily needed to get the spacecraft into orbit, not to stay in orbit.

In the above picture, you can see an illustration of a mission to the Moon. Rockets are conventionally built to have multiple stages. This is done because so much fuel is needed to get a payload from the surface to orbital altitude and accelerated to orbital speed. When about half the propellant is burned, the bottom half of the rocket is jettisoned. This makes the remaining rocket considerably lighter, which means when the engines in the next stage ignite, they will have a greater effect. The same can happen again when that next stage runs out of fuel. Eventually, all that’s left is the payload, traveling in the desired orbit. If that payload is carrying fuel, it is because it might have to do some other maneuver, such as a retro-fire to come home or a burn to transfer to another orbit, or it might need to control its attitude using small reaction control jets.